Relay cropping of cotton in wheat improves productivity of cotton-wheat cropping system

Cotton-wheat (CW) is an important cropping system in Pakistan; however, delayed cultivation of cotton after wheat significantly deceases system productivity. Late-sown cotton crop suffers from heat stress, high fruit shedding and infestation of various pests and cotton leaf curl virus (CLCV). Therefore, early sowing of cotton is preferred to overcome these chal-lenges. However, cotton planting is overlapped with the harvesting of wheat crop in CW cropping system. Since fallow land is unavailable, relay intercropping of cotton in wheat crop seems a viable option for early planting. This three-year field study evaluated the role of relay cropping of cotton in improving the productivity of CW cropping system and lowering CLCV infestation. Wheat was planted in 22.5 cm rows with no row skipped (W 1 ), one row skipped after every three (W 2 ), and six rows (W 3 ) for planting cotton in 0.75 and 1.50 m apart rows. Early cotton was planted in fallow land (C 1 ) and standing wheat in 0.75 (C 2 ) and 1.50 m (C 3 ) apart rows and conventional planting after wheat harvesting (C 4 ). Yield losses of wheat crop ranged from 8.16


Introduction
Cotton-wheat (CW) is the second most important cropping systems in South Asia after wheatrice spanning over an area of~4.19 mha in India and Pakistan [1]. The system provides cotton fiber, and wheat grains for food security and farmer's income. Generally, cotton cultivation is followed by wheat in CW system, and no fallow period is the most important issue of this system. The sowing of succeeding crop gets late because of overlapping harvesting period of preceding crop [2][3][4][5][6]. Late-sown cotton crop faces multiple stresses during different growth and developmental stages. The targeted cotton yield is not being achieved in Pakistan since the last two decades due to various biotic and abiotic constraints and all of which are responsible for delayed planting of cotton crop [7][8][9]. Generally, cotton is sown during mid-April and picked/harvested in October. However, late sowing results in impair seed germination, root shoot ratio, and seedling establishment due to high temperature [10,11]. Likewise, late planting pushes peak boll development period to cold weather which results in lower number of opened bolls. Although different agronomic approaches such as seed priming, mixed cropping or crop rotation have been employed to improve seed germination and production of cotton crop in CW cropping system, yield remains low. Several studies have demonstrated potential yield benefits for early planting in cotton crop [4,8,9,12]. Cotton leaf curl virus (CLCV) in Pakistan was initially reported in 1967 and its severity remained mild until 1990s. It drastically reduced total cotton production from 12.82 to 9.82 million bales within two years from 1991-92 to 1993-94 [13][14][15].
Several management and genetic improvement approaches have been employed since 1994 for reducing CLCV infestation in Pakistan. The peak CLCV infestation is recorded during 27 th to 31 st week of the year, regardless of the sowing time and genotypes [16]. Early planting enables cotton to complete its growth and boll setting during disease-free period. Meanwhile, late planted crop faces CLCV infestation at early growth stages, resulting in stunted growth. Several studies have concluded that early planting is a better choice for escaping CLCV infestation [9,17,18]. Furthermore, prolonged growth season and low CLCV infestation are the main contributor towards higher yield of early planted cotton [8,9,18]. Although development and cultivation of transgenic cotton (Bt-cotton) assisted in improving cotton production in Pakistan, it could not fulfill the demands of the country [14].
Besides high yield and disease escape, early planting got popular with the introduction of Bt-cotton in the country during early 2000s. It was due to inherent resistance against certain insects and improved boll setting ability of Bt-cotton over entire growth season [4,19]. Higher yield benefits, low disease infestation and better economic returns forced the farmers to keep land fallow during winter for spring planting of cotton. It was likely to reduce the wheat cultivation area in future and considered as a potential threat for national food security since >50% wheat comes from CW cropping system [4,20].
Relay cropping is one of the practical approaches to resolve the time conflict (3-4 weeks) between wheat harvesting and cotton sowing in CW cropping system [2,4,21]. Relay cropping is an intercropping system in which second crop is sown on onset of reproductive growth of first crop so that a full space is available after the harvest of the first crop [4,5,21]. It intensifies the system through time, and recommended method for areas characterized with short growing season [22]. The most important advantages of relay cropping are higher yield with efficient use of available resources from the same piece of land in a specific time [5]. The basic objective of relay cropping is to extend the growing season of second crop by early planting. However, it is important to develop strategy for minimizing the damage to first crop during sowing and to second crop while harvesting first crop [23]. Potential benefits of relay intercropping of wheat-potato [24], rice-chickpea [25], red clover-wheat [26], legumes-millet [27], legumes-maize [28], lentil-rice [29] and muskmelon-strawberry [30] have been previously documented.
Initially, relay cropping in cotton-wheat cropping system was implemented by broadcasting wheat seed in standing cotton crop [2,3,6,31]. Since early 2000s, the feasibility of various techniques to plant cotton in standing wheat is being investigated in China and Pakistan [4,20,[32][33][34]. The techniques used so for are labor-intensive and unconducive to mechanization. Moreover, potential of relay cropping as management strategy for CLCV in CW cropping system and its efficiency have not been investigated yet. We hypothesize that relay cropping of cotton in skipped wheat rows may improve economic returns and lower CLCV infestation. To do this, some modifications in wheat sowing pattern such as managing row spacing and skipping rows are required for successful planting. In Pakistan, wheat is drilled at 22.5 cm apart rows and cotton is planted in 0.75 m apart rows. Therefore, wheat row can be skipped at specified distance to make cotton planting convenient. The experiment was designed by skipping the cultivation of wheat in few rows during winter season and cotton was relay planted in skipped rows during summer season to overcome problems associated with delayed sowing. The main objectives of the study were to lower CLCV disease through early planting, use of minimum tillage and improving profitability CW cropping system.

Soil and weather
Three years (2013-16) field study was conducted at agronomic research area of Central Cotton Research Institute (CCRI), Multan, Pakistan (30˚12'N, longitude 71˚28'E and altitude 123 m a. s.l). The soil of the experimental site was alkaline (pH = 8.1), poor in organic matter (1.1%) and silt-loam in texture with good drainage. The metrological data were recorded by weather station in the vicinity (300 m) of experimental site. The climate of experimental area was arid due to high evaporation rate, mean maximum (30.01˚C) and minimum daily temperature (19.96˚C). The daily mean relative humidity and total average annual rainfall was 71.64% and 255.26 mm, respectively (Table 1).

Wheat crop husbandry
Wheat seeds were drilled in 22.5 cm spaced rows by using 120 kg seed per hectare on 28 th November, 8 th December and 28 th December in 2013, 2014 and 2015, respectively. The treatments were conventional wheat sowing at 22.5 cm space (W 1 ), every 4 th (W 2 ) and 7 th wheat row skipped (W 3 ) for making cotton planting convenient in standing wheat. Wheat crop was fertilized with 125 kg N ha -1 in three splits (pre-sowing, first irrigation and booting stage) and 100 kg P 2 O 5 ha -1 as pre-sown application. The 4 th and 7 th row was skipped for cotton planting in 0.75 and 1.50 m apart rows, respectively. This made 22.5 cm space available for cotton planting between rows, while space occupied by wheat was used as tractor track and implements after wheat harvesting. A 1 m 2 area was manually harvested at maturity during end of April to record grain yield of wheat crop.

Cotton crop husbandry
The transgenic cotton cultivar ('CIM-616') was planted as monoculture on fallow land for C 1 , in standing wheat at 0.75 m (C 2 ) and 1.50 m distance (C 3 ) and conventional planting after wheat harvesting (C 4 ). It resulted in a ratio of one row of cotton in three (1:3) and six (1:6) rows of wheat in C 2 and C 3 , respectively (Fig 1).
Light irrigation was applied to wheat field before cotton sowing as relay crop (C 2 and C 3 ). i.e., wheat phase, wheat-cotton intercrop, and cotton phase. The crop was fertilized with 150 kg N ha -1 from urea by fertigation methods in four equal splits. The wheat and cotton shared about 5 weeks of overlapped growth period where reproductive development of wheat was overlapped with seedlings stage of cotton. The seed cotton yield was recorded by manual picking of fully opened bolls. Data on bolls (m -2 ) and boll weight were recorded from three randomly selected places and hundred opened bolls, respectively. The CLCV incidence was recorded periodically at 15-days interval from 1 st June to 30 th September and presented as incidence (%).

Economic and statistical analysis
The land use, production efficiency and profitability were calculated. Land use efficiency was calculated by dividing the number of cropping days to the total number of days in a year. The production efficiency was determined by dividing total production of cropping system to the total number of days of a year. The formula for land use and production efficiency are given below [35] [36] The D1 and D2 are number of cropping days of first and second crop in a cropping system The Y1 and D1 are yield and duration first crop, while Y2 and D2 are yield and duration of second crop, respectively.
The profitability (income per day) was worked out by following the procedure of Mukherjee [37].
Profitability US$ day À 1 ha À 1 ð Þ ¼ Total net income of cropping system Total cropping duration The economic analysis was carried to determine the profitability of cotton wheat relay cropping including all the expenses from sowing to harvesting of both crops. Benefit cost ratio (BCR) was estimated using prevailing average cost of inputs and value of produce. Statistix 8.1 software was used for analysis of variance and means comparison was carried out through least significant difference test (LSD) at 5% probability level [38].

Yield and related traits of wheat and cotton
The results indicated that number of productive tillers and grain yield of wheat were significantly affected by different planting treatments, whereas number of grains per spike and 1000-grain weight remained unaffected (Fig 2). The highest number of productive tillers were recorded in W 1 followed by W 3 and W 2 . Wheat sown in W 2 and W 3 produced 19.5 and 9.2% lesser grain yield than W 1 during 2014. Significant reduction in number of productive tillers was observed in 2015. The tillers were reduced by 19.9% and 9.5% in W 2 and W 3 compared to W 1 . Meanwhile, grain yield was decreased by 20.3% in W 2 and 8.9% in W 3 during 2015 in comparison with W 1 . The number of productive tillers and grain yield were reduced by 23.2 and 11.4%, and 19.6 and 6.4%, respectively during 2016.
There were significant variations among planting treatments for plant height, number of bolls and seed cotton yield (Fig 3). Similar plant height was noted for cotton sown in C 1 , C 2 and C 3 during 2014; however, it was higher than C 4 . Relayed cropped cotton in C 2 produced similar number of bolls (m -2 ) and seed cotton yield with C 1 . Cotton sowing in C 4 resulted in the lowest number of bolls and seed cotton yield. Plant height, bolls, and seed cotton yield in C 4 were statistically lower than rest of the treatments. However, C 1 , C 2 and C 3 were non-significant with each other for plant height. The C 1 and C 2 produced statistically similar number of bolls and seed cotton, which were higher than C 4 and C 3 . The similar results as of 2014 were recorded during 2015 and 2016. The yield was improved by 89.7%, 89.6% and 68.7% in C 1 , C 2 and C 3 , respectively over C 1 during 2016. The results of three years experimentation showed that relay cropped cotton in C 2 produced equal yield with C 1 indicating that C 2 can be adopted as an alternative to C 1 .
The CLCV infestation was significantly influenced by different planting techniques used in the study. The differences in CLCV infestation during 2014 were non-significant in C 1 , C 2 , C 3 ; however, these treatments observed low CLCV infestation compared to C 4 . Similar trend was observed during 2015. Moreover, relay cropped cotton (C 2 and C 3 ) observed the lowest CLCV infestation during 2016. The highest CLCV infestation during 2016 was noted for C 4 (Fig 4).  Table 2).
The data further revealed that loss wheat grain yield loss in W 2 and W 3 for cotton relay cropping should not be considered as a significant economic loss since it can be compensated from higher seed cotton yield which has two times high unit price than wheat. The performance of various planting treatments was in term of economics, land use and production efficiency are given in Table 3. The total cropping duration and system productivity differed across various treatments. The highest duration, land use and production efficiency were recorded for relay cropping treatments W 2 C 2 and W 3 C 3 , whereas monocrop cotton resulted in the lowest values for these parameters. Cotton monocrop (C 4 ) recorded higher values for   W 1 C 4 = conventional wheat-cotton sowing, W 2 C 2 = wheat-cotton relay cropping system (R×R = 75 cm), W 3 C 3 = wheat-cotton relay cropping system (R×R = 150 cm), C 1 = cotton as sole crop (R×R = 150 cm),-indicates that computation was not possible.
daily profitability because it is only for cotton season (about 8 month) rather than whole year as for other treatments.

Discussion
Different planting techniques exerted significant impacts on crop yields and productivity of CW cropping system in the current study. Wheat sowing according to W 2 and W 3 produced less grain yield than W 1 since a significant portion of land was left unseeded due to skipped rows for cotton planting between rows. It reduced overall plant population, density of productive tillers and grain yield. However, yield loss was comparatively less in W 3 than W 2 due to higher plant population. Despite 25 and 14% reduction in number of wheat rows in W 2 and W 3 , the corresponding yield reductions was 20.3 and 9.2%, respectively. Relatively low yield reduction in comparison with reduction is number of rows is due to higher tillering in outer rows. It compensates the yield losses to some extent. It has been reported in literature that yield of companion crop is decreased in intercropping due to strong competition for natural resources [39]. However, the yield reduction in the current study was mainly due to low plant population because of skipped wheat rows. Low wheat grain yield in relay cotton and maize has been witnessed in earlier studies [4,40]. Relative low grain yield was recorded during 2016 because of late sowing and prevalence of relatively high temperature during March (Table 1) which impaired normal development of grains. Cotton planting in wider rows availed equal growth period; however, produced lower yield due to sub-optimal plant population. Although C 3 had half number of rows compared to C 2 , reduction in number of bolls and seed cotton yield was only 13.4 and 12.3%, respectively. The plants from outer rows availed more space and produced long branches to compensate yield losses caused by a smaller number of rows. Significant yield differences can be linked with the extra two months growth period resulting from early planting. Crop sown during the end of March or early April attained significant height before the onset of heat stress during June and July (Table 1) and CLCV incidence which is quite clear from Fig 4. Irrespective of crop age, CLCV incidence is favored by small differences in day and night temperature. Such conditions are naturally available from mid-July to end of August (Table 1). The early planted cotton had sufficient CLCV-free growth period and better tolerated its infestation in the current study. Previous studies also reported low CLCV infestation on early sown crop [9,18]. Yield of relay cropped cotton (C 2 and C 3 ) ranged from 3992.9 to 4505.7 kg ha -1 corresponding to 0.37 and 11.71% decrease compared to C 1 . On the other hand, C 2 and C 3 produced 86.07 and 64.9%, more seed cotton yield, respectively compared to C 4 . Higher seed cotton yield in fallow land and relay cropping treatments due to prolonged growth period to produce more bolls and sufficient time for boll opening. Overall cotton performance in 2015 was poor due to excessive rainfall (Table 1).
Higher seed cotton yield in cotton relay cropping is in line with earlier studies [4,19,41,42]. In contrast to our results, the lower seed cotton yield has been reported over sole crop in China due to poor light availability [33]. However, light availability was not an issue in the current study and standing wheat provided protection to young cotton plants from hot wind. Therefore, yield of relay system was not less than fallow land planting (Fig 3). Relay cropping of cotton although produced statistically similar yield, but the technique also permits the cultivation of wheat which is an additional benefit in contrast to sole cotton sowing.
Economic analysis is important for the social acceptance of relay cropping among the farmers [43][44][45]. Better economics returns, land use and production efficiency of cotton relay cropping was due to timely planting and low production costs, mainly invested in conventional seedbed preparation [4]. The cost of cultivation for wheat varied for sowing, irrigation, weeding, plant protection and picking (Table 3). Although cotton planting after wheat harvesting had less economic benefit over cotton planting as sole, exclusion of wheat is not a good decision national food security. Data in Table 2 revealed better economic return of cotton-wheat cropping system were based on existence of cotton either its sole cultivation or relay cropped with wheat. The higher cotton yield in relay cropping also compensate the economic losses of low yield from skipped wheat rows for cotton sowing as relay. The decision of producing only cotton is good if higher market prices exist. The unit price of cotton varies greatly from year to year and decision of cotton planting after wheat or fallow land depends on the cotton prices. Considering the greater flexibility in cotton prices and wheat role in national food security, it is recommended that exclusion of wheat from system should not be compromised, where relay cropping is potential tool for ensuring higher profitability and food security [20].

Conclusions
Skipped rows planting system of wheat decreased its yield; however, resulted in higher yield of cotton. It was observed that relay planting did not reduce the cost of cultivation but likely to improve the productivity of cropping system. Although, skipped rows in wheat produced less yield but maintained the higher returns round the year through higher cotton yield. Furthermore, relay cotton planting in late March is a better choice to manage cotton leaf curl virus infestation and increase system productivity by extending the growing season rather to exclude wheat by keeping land fallow for early planting of cotton. It ensures good economic returns and higher system productivity in comparison to conventional planting system after wheat harvesting which is characterized with low yield due to high cotton leaf curl virus attack.